Method for developing electrostatic latent images by utilizing coupling reaction

ABSTRACT

A method for developing an electrostatic latent image into a visible image by utilizing a coupling reaction, said method comprising immersing a recording or a copying material such as an electrostatic recording material or an electrophotographic copying material carrying an electrostatic latent image in a liquid developer comprising a carrier liquid consisting of a paraffinic hydrocarbon having a high-electric resistance of not lower than 109 Omega . cm and a low-dielectric constant of not greater than 3 and a diazo compound or toner (charged particles) dispersed in said carrier liquid and having its polarity regulated by a polarity regulating agent consisting of a synthetic resin, and thereafter bringing the resulting recording or copying material into contact with a vapor of a coupler adapted to form an azo dye by reaction with said diazo compound or by contact with an aqueous solution containing such a coupler, whereby forming a visible image on the surface of said recording or copying material.

United States Patent [72] Inventors Zenllro Okuno;

Shuuiti Mlyabe, both of Tokyo, Japan [21 Appl. No. 751,758 [22] Filed Aug. 12, 1968 [45] Patented Nov. 23, 1971 [73] Assignee Kabushiki Kalsha Ricoh Tokyo, Japan 32 1 Priority Aug. 27, 1967 l J p n [3 l 42/53850 [54] METHOD FOR DEVELOPING ELECTROSTATIC LATENT IMAGES BY UTILIZING COUPLING REACTION 5 Claims, No Drawings s2 U.S.Cl 117/37 LE, 96/49, 101/469, 252/621 [51] Int. Cl ..G03g13/10, 003g 9/04 [50] Field oiSearch 117/37 LX; 252/627; 101/469; 96/49 [56] References Cited UNITED STATES PATENTS 2,597,306 5/1952 Eaton ct al 8/71 2,877,133 3/1959 Mayer 117/37 3,083,117 3/1963 Schmiedelet al 117/17.5 3,157,546 ll/i964 Cover 156/231 3,253,913 5/1966 Smith et ai 117/37 X 3,259,581 7/1966 Matkan 252/621 3,272,644 9/1966 Nelson 1 17/37 X Dinaburg, M. S., Photosensitive Diazo Compounds and their Uses" Focal Press translated from a 1964 Russian book, page 58.

Primary Examiner-William D. Martin Assistant Examiner-Edward J. Cabic Attorney-Woodhams, Blanchard and Flynn ABSTRACT: A method for developing an electrostatic latent image into a visible image by utilizing a coupling reaction, said method comprising immersing a recording or a copying material such as an electrostatic recording material or an electrophotographic copying material carrying an electrostatic latent image in a liquid developer comprising a carrier liquid consisting of a parafiinic hydrocarbon having a high-electric resistance of not lower than 10 0. cm and a low-dielectric constant of not greater than 3 and a diazo compound or toner (charged particles) dispersed in said carrier liquid and having its polarity regulated by a polarity regulating agent consisting of a synthetic resin, and thereafter bringing the resulting recording or copying material into contact with a vapor of a coupler adapted to form an azo dye by reaction with said diazo compound or by contact with an aqueous solution containing such a coupler, whereby forming a visible image on the surface of said recording or copying material.

METHOD FOR DEVELOPING ELECTROSTATIC LATENT IMAGES BY UTILIZING COUPLING REACTION BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention is concerned with a method for developing an electrostatic latent image into a visible image by utilizing the so-called coupling reaction, said method comprising immersing either an electrostatic recording material or an electrophotographic copying material after same has been processed to carry an electrostatic latent image (such a recording or copying material will hereinafter be referred to simply as an electrostatic charge carrier) in a liquid developer containing a diazo compound having a regulated polarity, and thereafter bringing the resulting electrostatic charge carrier into contact with a coupler to thereby form a copied image which bears a desired color.

2. Description of the Prior Art The development of electrostatic latent images has been performed by relying on the following two types of developing methods, one of which is called the wet process development and the other the dry process development. The former of these two types of conventional developing methods comprises immersing an electrophotographic copying material or an electrostatic recording material already carrying an electrostatic latent image thereon in a liquid developer prepared by dispersing a kneaded mixture of particles of a black pigment, such as carbon black, and additives, such as a polarity regulating agent and a dispersing agent, in a carrier liquid having a volume resistance of not lower than 0.. cm. and a dielectric constant of not greater than three to deposit, by electrophoresis, said particles of pigment onto the area of the latent image carried on said copying or recording material, thereby developing the latent image into a visible image. The latter of said developing methods performs the development of an electrostatic latent image by electrically causing a charged toner powder to be adsorbed onto the area of the latent image formed on the aforesaid copying material or on a light-sensitive plate. When latent images were developed by relying on these developing methods of the prior art, however, the resulting copied images were obtained in black color in most cases, and thus, the copied images have been uninterestingly monotonous in the effect. Also known is a different type of developing method, which is designed so as to form the so-called colored copied images by the use of a variety of wet process developers prepared with pigments of various colors such as black, yellow, blue and red and by repeating the procedure of imparting an electrostatic latent image onto a sheet of copying material and by repeating the developed process utilizing the aforesaid variety of developers each containing a pigment which differs in color from the rest of them.

In each of these known developing methods, the basic procedure of forming a copied image invariably consisted of depositing, by electrophoresis, the particles of colored pigment onto the area of the electrostatic latent image formed on :1 copying material. In view of the fact, however, that these particles of pigment were strictly required to possess the various conditions, i.e. the various physical as well as the chemical properties as well, as being the basic component material constituting the toner, the kinds of colors of the pigment particles were inevitably limited to a very narrow range. Accordingly, the colors of the copied image obtained were limited to only those monotonous tones.

SUMMARY OF THE INVENTION It is, therefore, the object of the present invention to provide a method for developing an electrostatic latent image to form a so-called colored copied image which may be either in a single color or superimposed synthetic colors or combinations thereof.

The present invention concerns a developing method utilizing the so-called coupling reaction between a diazo compound and a coupler. According to the present invention, it is possible to form copied images in a variety of different colors which can be selected from an extensive range of colors.

The electrostatic latent image which is developed according to the present invention may consist of any latent image so long as it is one that is formed by an electrostatic charge impressed on an electrostatic charge carrier. The present invention can be applied to all of those electrostatic latent images which are formed by the so-called electrostatic charge impressed on electrostatic charge carriers such as those comprising a photoconductive layer which is provided on a support, or those carriers comprising an insulated layer which is provided on a support, or those carriers which consist of nothing else but insulating materials alone.

On the other hand, the present invention permits the use, as the basic material for constituting the toners of the developers, all of those diazo compounds which have been used conventionally as an important component of the lightsensitive layers of the diazo-type copying materials of the prior art.

The developer which is used in the present invention is not capable, by nature, of forming a visible image when it is used independently. But, according to the present invention, this developer points to such a solution that is prepared by dispersing-in a carrier liquid consisting of a hydrocarbon having a high electric resistance of not lower than 10 9. cm. and a dielectric constant of not greater than three-any one of the toners selected from the group of toners consisting of the kneaded mixtures of diazo compounds and polarity regulating resins, the kneaded mixtures of diazo compounds, polarity regulating resins and surface active agents and those toners which will be described later in this specification.

The aforesaid kneaded mixtures or toners are obtained in the following manner. Namely, a diazo compound such as 2,5- dimethoxy-4(4-methylthiophenol) diazonium chloride, 2,5- diethoxy-4(4-methoxybenzoylamide) diazonium chloride and 4N,N-ethylhydroxyethyl diazonium chloride is kneaded with a synthetic resin, such as lauryl methacrylate-acrylamide copolymers, lauryl methacrylate-dimethylaminoethyl methacrylate copolymers and styrene-acrylicester copolymerswhich have been used as the agent for regulating the polarity of the toners of the conventional wet process developers, in the presence of an aromatic solvent or a solvent of petroleum origin such as naphtha. In the present invention, there are used those developers which are prepared by dispersing the aforesaid kneaded mixtures in carrier liquids consisting of paraffinic hydrocarbons having a volume resistance of not lower than I0 9. cm. and a dielectric constant of not greater than three.

As the carrier liquids, those of the aforesaid paraffinic hydrocarbons having the foregoing properties, such as ISOPAR H (the trade name of the product manufactured by Esso Standard Oil Company of U.S.A.), ISOPAR G, ISOPAR L (both are the products of the same company), Naphtha No. 6 (t rade name of the product manufactured by Esso Standard Oil Company), are suitable for use in the present invention. However, halogen-bearing hydrocarbons such as Ereon of E. I. du Pont de Nemours & Co., Inc. of USA. can be used also in the present invention.

In the wet process developers, in general, which are intended for the development of electrostatic latent images, the charged toners which are contained therein have been required to possess a surface potential in the range of i60 mv. Those developers which are prepared by dispersing only said diazo compounds in carrier liquids consisting of parafiinic hydrocarbons have resulted in that the particles of said compounds exhibit a surface potential of only several mv., and accordingly, these developers will never satisfy the practical purposes. In case, however, said diazo compounds which have been kneaded with a polarity regulating agent consisting of an aforesaid synthetic resin are dispersed in carrier liquids, the resulting developers will show a surface potential of not lower than 60 mv. and, accordingly, these developers will sufficiently satisfy the practical purposes.

In the present invention, there is used also the developer of the type which is prepared by first mixing a diazo compound with a white fine powder such as silica powder by the use of an appropriate solvent such as water or an organic solvent such as alcohol, kneading the resulting particles-which are in the form that the diazo compound has adsorbed to the particles of the silica powdertogether with an aforesaid polarity regulating agent, and dispersing the kneaded mixture in a carrier liquid. This type of developer is more desirable than the aforesaid developers because, when it is used in the developing process, the copying material on which a copied image is formed through contact with the coupler will have the advantage that the background of the developed copying material is prevented from being soiled. Furthermore, there also can be used the developer which is prepared by dispersing, in a carrier liquid, a compound formed by chemically introducing a diazonium salt in the phenyl nucleus of styrene contained in a styrene-bearing multiple copolymer and by causing said compound to be adsorbed onto a white fine powder. In such an instance, the introduction of the diazonium compound in the phenyl radical of styrene can be carried out either by the nitration of the styrene polymer or by polymerizing a paraaminostyrene as a copolymeric monomer, and then diazonizing the resulting polymer. In this same instance, by preparing a copolymer having a composition in which the diazo compound occupies the predominant proportion, it is possible to use this copolymer directly as the toner without the need of having it adsorbed onto the white fine powder. In case a polyvalent metal salt of an organic acid such as manganese naphthenate, nickel naphthenate, cobalt naphthenate and calcium oleate is added to such a developer which can be applied to the present invention, the resulting developer will have a polarity which is regulated to the positive charge, with the surface potential being in the range of+60 +80 mv.

More specifically, the toner which is contained in the developer which can be applied to the present invention may consist of any one of the following group:

a. toners consisting of a mixture of a diazo compound and a polarity regulating agent which consists of a synthetic resin;

b. toners prepared by first having a diazo compound adsorbed onto a white fine powder and then kneading the resulting substance together with a polarity regulating agent;

c. toners consisting of a compound prepared by chemically uniting a diazo compound to a polarity regulating agent such as a styrene-bearing copolymer;

d. toners prepared by having the compound of(c) adsorbed onto a white fine powder; and

e. toners consisting of a kneaded mixture of any one of the toners described in (a) through (d) and a surface active agent.

The mixing or uniting ratio between the diazo compound and the polarity regulating agent in the foregoing toners is desirably one to 50 parts by weight of the polarity regulating agent relative to one part by weight of the diazo compound. The mixing ratio of the diazo compound to the white fine powder is desirably parts by weight of the white fine powder relative to one part by weight of the diazo compound. By setting the mixing ratio of said diazo compound to the surface active agent so as to be 0.0l-0.05 part by weight of the surface active agent relative to one part by weight of the diazo compound, the toner will have a polarity which is regulated clearly to the positive charge.

As the synthetic resins for use in regulating the polarity of the toners, those acrylates, other than the aforesaid acrylate copolymers, such as the commercially available Plexrol 996 (the trade name of the product of Rhorn & Hass Co. of U.S.A., which is a nitrogen-bearing polymer), alkyd resins, polymers linseed oil and rosin are desirable also.

As the couplers which are adapted to form azo dyes by uniting to the aforesaid toners containing diazo compounds, there are those which produce blue dyes and which include 2,3- hydroxynaphthalene, 2-hydroxy-3,6-disulfonic acid and 2- amino-S-hydroxynaphthalene-7-sulfonic acid monourea, and there are those which form red dyes and which include 3- methylphrazolone, 3,5-diethoxyphenol, l,3,5resorcinic acid anilide, 5,5-dimethyl-l,3-cyclohexanediene and l-p-sulfophenyl-3-methyl-5-pyrazolone, and still there are those which form yellow dyes and which include acetoacetoanilde, 2,5-dimethyl-4-morpholinomethylphenol, l-methyl-3hydroxy-4-acetoamidebenzene and 2-hydroxy-4-methylphenyl. Those couplers which form violet dyes include, other than the couplers employed in the examples which will be described later, 3-acetoacidphenol, 3-(aimnophenylsulfonylamino)- phenol, 2-(3-aminobenzeneamide)-7-naphthol and 7-hydroxynaphthalene-l-biguanidine-HCl. Other than the foregoing couplers, there are resorcin (which form a brown dye), lphenyl-3-methyldiazole (which forms an orange dye), phloroglucin (reddish violet dye), monohydroxethylether of catecol (yellowish brown dye), phloroglucide (violet black dye) and 2,3-dihydroxynaphthalene monoglycollic acid either (blue black dye).

According to the development method of the present invention, an electrostatic charge carrier (a carrier of an electrostatic image)which consists of an electrophotographic copying material having been processed in such a way that, after is it impressed with the positive or the negative charge by a known method, it is exposed to light rays through a negative or a positive original or which consists of an electrostatic recording material having been impressed directly with an electrostatic latent image-is immersed in a developer as those described above, so that, as the result, the toner which comprises a diazo compound is deposited, by electrophoresis, onto the area of the electrostatic latent image formed on said an electrostatic charge carrier such as copying or recording material, and then the resulting electrostatic charge carrier is either immersed in an alkaline solution containing an appropriate coupler elected from the aforesaid group and being capable of forming a dye of a desired color through its reaction with the aforesaid diazo compound, or is brought into contact with the vapor of a sublimable coupler such as resorcin or phloroglucin to thereby produce a dye bearing the desired color.

The formation of a dye according to the present invention is performed by relying on any one of the aforesaid roughly divided two types of color developing procedures. in case, however, the procedure of color developments relies on the use of said solution, it is desirable to perform this color development after coating the color-developing face of the copying or the recording material with a thin film ofa synthetic resin in order to avoid the dissolution of the diazo compound which has been electrically deposited on the area of the electrostatic latent image formed on the electrostatic charge carrier into the solution containing said coupler. As the coating agents, the employment of a solution of an organic solvent, such as toluene and acetone, which contains, in the form of being dissolved therein, any one of the following substances selected from the group consisting of acrylates such as methyl methacrylate, cellulose derivatives such as cellulose acetate, styrene and its copolymers is desirable.

By the adoption of the developing method as has been described above, it is possible to form a so-called colored copied image which is provided in the form of a combination ofa multitude of different colors either presenting individual color effects or presenting superimposed synthesized color effects. For example, in case a colored original which is prepared in the three different colors of blue, red and yellow is to be copied by the use of an electrostatic charge carrier provided with a photoconductive layer, said carrier which has been charged by a known charging process is exposed to light rays through said original and also through a yellow filter which is complementary to blue color and then the resulting exposed carrier is brought into contact with a developer of the present invention, and thereafter the resulting carrier is subjected to a color developing process by the use of 2,3-hydroxynaphthalene to produce the region of the blue copied image first. Then, the resulting carrier is again charged and exposed to light rays through the original and through a bluish green filter and thereafter it is brought into contact with the developer, followed by the formation of the region of the red copies image by the use of 3-methylpyrezolone Finally, the carrier which has undergone the foregoing two steps of color development is subjected to a similar series of procedures, i.e. charging--exposure to light (using a blue filter)bringing into contact with the developer-formation of the region of the yellow copied image by the employment of acetoacetoanilide. As a result, a colored copied image is obtained.

As has been described, according to the developing method of the present invention, not only is there obtained a copied image bearing colors of a very wide range, but also is there provided an advantage that the copied image is of a satisfactory clarity. Thus, the developing method of the present inven tion is indeed epoch-making.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will hereunder be described in further detail in connection with some of the embodiments which are given simply by way of example. While these embodiments employ the electrophotographic process as the means of forming electrostatic latent images, it should be understood that the means of forming electrostatic images is not limited only thereto and also that those latent images which are formed on insulated papers by directly impressing then with an electric charge can be equally efl'ectively developed.

EXAMPLE 1 After thoroughly mixing, by stirring, 3 gr. of a diazo compound which consisted of 2,3-dimethoxy-4(4- methylthiophenol) diazonium chloride expressed by the structural formula:

OCH:

CHzO

1.2 gr. of Plexrol 996 (the trade name of the product of Rhom and Hass Co., a nitrogen-bearing polymer), 2gr, of the resulting mixture was dispersed in 300cc. of an isoparaffinic hydrocarbon (a product manufactured and sold under the trade name of lSOPHAR H by Esso Standard Oil Company), and thus a developer containing a toner comprising a diazo compound and having a polarity regulated to the negative charge was prepared.

After having impressed a commercial electrophotographic copying material (whose photoconductive layer was comprised of zinc oxide and a synthetic resin) with a negative charge by known corona discharge, the resulting copying material was exposed to light rays coming from a tungsten lamp through a negative original to thereby form an electrostatic latent image thereon. Thereafter, the resulting copying material was immersed in the aforesaid developer for lseconds. As a result, the diazo compound was deposited electrically on the area of the electrostatic latent image, and thus a copying material on which a light yellow positive copied image was formed was obtained.

On the other hand, together with 1 gr. of potassaium hydroxide, 5 gr. of phloroglucin was dissolved in 100 cc. of methanol, and thus, a coupler solution was prepared. In this solution was immersed the copying material carrying a light yellow positive copied image which was formed by the electrical deposition of said diazo compound to cause a reaction between this diazo compound and the coupler. As a result, a violet positive copied image was obtained.

EXAMPLE 2 3.0 gr. of a diazo compound which consisted of 2,3 diethoxy- 4(4'-methoxybenzoyamide) diazonium chloride expressed by the general fonnula:

1.2 gr. of Plexrol v 966 (a nitrogen-bearing polymer) and Ice. of isoparafiinic hydrocarbon solution of lpercent manganese naphthenate were kneaded in a mixing bowl. Two grain of the resulting kneaded mixture was dispersed in 300cc. of lSOPAR H, and thus, a developer containing a toner comprising a diazo compound and having a polarity regulated to the positive charge was prepared. 2

After impressing an electrophotographic copying material with the negative charge in the same manner as that in example l and then exposing this copying material to light rays through an original carrying a black positive image, the resulting copying material was immersed in the developer obtained in the manner described, or l0seconds. As a result, the diazo compound was electrically deposited on the area of the electrostatic latent image formed on the copying machine and thus a copying material carrying a light yellow positive copied image was obtained. The resulting copying material was then brought into contact with the phloroglucin vapor which was produced by subliming the phloroglucin by heating it. As a result, a dark violet positive copied image was obtained.

EXAMPLE 3 Onto the face of the copying material obtained in example 2 and carrying a light yellow positive copied image formed by the electric deposition of the diazo compound was lightly sprayed a 3percent methylmethacrylate/toluene solution by a spray gun to form a coat thereover. Thereafter, the resulting copying machine was brought into contact with phloroglucin vapor. Thus, a beautiful dark violet positive copied image which was free of soiled background was obtained.

EXAMPLE 4 1.2 gr. of a diazo compound which consisted of 2,3- dimethoxy-4(4'-methylthiophenol)- diazonium chloride, 1.2 gr. of fine silica powder and cc. of methanol were mixed together in a ball mill. Thereafter, the resulting mixture was subjected to suction filtering in darkness. The residue of the filtering was dried under a reduced pressure. Thus, a white powder of the type that the diazo compound was adsorbed onto the fine silica powder was prepared. 2.4 gr. of this powder, 1.2 gr. of a resin consisting of a lauryl methacrylatedimethylaminoethyl methacrylate (9: l), 2 cc. of solution of lpercent manganese naphthenate (manganese content being percent) in ISOPHAR H and 40 cc. of ISOPHAR H were kneaded together in a porcelain ball mill for 8 hours. Two grain of the resulting mixture paste was dispersed in 300 cc. of lSOPHAR H, and as a result, a developer containing a toner comprising a diazo compound and having a polarity regulated to the positive charge was prepared. In this developer were immersed, for 20 seconds, 4electrophotographic copying materials carrying electrostatic latent images which were formed 70 thereon in a manner similar to that described in example 2,

and as a result, 4copying materials each carrying a light yellow positive copied image which was formed by the electrical deposition of the diazo compound were obtained.

Thereafter, the respective copying materials carrying positive copied images were brought into contact with the following four kinds of vapors of couplers for a period of time ranging from to lOseconds, respectively:

copolymers, styrene-acrylic ester copolymers, nitrogenbearing acrylate polymer, alkyd resins, polymerized lin- Color of the final copied image Yellow.

Orange.

Brown.

Reddish violet.

As a result, 4positive copied images of yellow, orange, brown,

and reddish violet in color, corresponding to the respective couplers, were obtained.

EXAMPLE 5 ogr. of para-aminostyrene and 135 gr. of butyl methacrylate made into the form of suspended fine particles. While keeping this suspension at 0 C, a mixture of 0.05 mole of sodium nitrite and 0.05 mole of hydrochloric acid was introduced therein during 2 hours. By filtering out the resulting substance,

para-diazostyrene-para-aminostyrene-butylmethacrylate copolymer (a 23percent diazonified relative to the amino radical) was obtained. Under a reduced pressure, this copolymer was dried at normal temperature. and as a result, a yellowish brown powder was obtained.

3 grain of this powder and 005 gr. of nickel naphthenate were mixed, together with 100 cc. of a solvent of petroleum 5O origin (Naphtha No. 6), in a ball mill, and thus, a liquid developer was prepared. This liquid developer comprised a toner which was positively charged. When a copying material bearing an electrostatic latent image was dipped in said developer in a manner as described in example 1, there electrically deposited particles of a yellowish brown diazo compound onto the area of the latent image which consisted of a negative charge. By applying the vapor of phloroglucin to the resulting surface, there was obtained a blackish-brown copied image was formed.

We claim: 1. A method for developing an electrostatic image, comprising the steps of:

sisting of lauryl methacrylate-acrylamide co olymers, laury methacrylate-dimethylaminoethyl met acrylate seed oil and rosin; (b) a toner prepared by first adsorbing a diazonium compound onto fine silica powder and then kneading the resulting substance together with said polarity regulating agent; (c) a toner consisting of a compound prepared by chemically introducing a diazo compound in the phenyl nucleus of styrene contained in a styrene-bearing copolymer polarity regulating agent; (d) a toner prepared by having the toner of (c) adsorbed onto said fine silica powder; and (e) a toner consisting of a kneaded mixture of any one of the toners (a) through (d) and a surface active agent selected front the group consisting of manganese naphthenate, nickel naphthenate, cobalt naphthenate and calcium oleate; and then bringing said surface of the resulting electrostatic charge carrier on which said latent image is formed into contact with an azo-coupling compound capable of coupling with said diazonium compound to form a visible image consisting of an azo dye on said surface of the electrostatic charge carrier 2. A method according to claim 1, wherein an alkaline solution of said azo-coupling compound is brought into contact with said electrostatic charge carrier to order to form said visible image.

3. A method according to claim 1, wherein said azocoupling compound is selected from the group consisting of acetoaceto-anilide, l-phenyl- S-methyldiazole, resorcin and phloroglucin, and said azo-coupling compound is brought into contact with said electrostatic charge carrier in order to form said visible image.

5. A method according to claim 1, wherein said diazonium compound is selected from the group consisting of 2,5 dimethoxy-4-(4-methylthiophenol) -benzene diazonium chloride, 2,5-di-ethoxy-4-(4-methoxybenzoylamide) benzene diazonium chloride and 4-N,N-ethylhydroxyethyl benzene diazonium chloride.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 622 368 Dated November 23, 1971 Inventor(s) Zenjiro Okuno and Shuuiti Miyabe It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, line 38; change "front" to from-.

Column 8, line 55; after "compound" add is sublimed by heating it and the resulting vapor thereof.

Colunn 8, line 68; change "in" to to.

Signed and sealed this and day of May 1972.

(SEAL) Attest:

EDWARD M.FLE'I'CHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents RM PO-105 (10-69) USCOMM-DC 00376-F'69 U 5 GOVERNMENT PRINTING OFFICE l9. 0-365-834 

2. A method according to claim 1, wherein an alkaline solution of said azo-coupling compound is brought into contact with said electrostatic charge carrier to order to form said visible image.
 3. A method according to claim 1, wherein said azo-coupling compound is selected from the group consisting of acetoaceto-anilide, 1-phenyl-3-methyldiazole, resorcin and phloroglucin, and said azo-coupling compound is sublimed by heating it and the resulting vapor thereof is brought into contact with said electrostatic charge carrier in order to form said visible image.
 4. A method according to claim 1, wherein after said electrostatic charge carrier is immersed in said liquid developer and prior to contacting said charge carrier with said azo-coupling compound there is sprayed onto the surface of said electrostatic charge carrier on which said electrostatic latent image is formed, an organic solvent solution containing dissolved therein a compound selected from the group consisting of methyl methacrylate, cellulose acetate, styrene and styrene-bearing copolymers, following which a solution of said azo-coupling compound is applied in the charge carrier.
 5. A method according to claim 1, wherein said diazonium compound is selected from the group consisting of 2,5-dimethoxy-4-(4'' -methylthiophenol)-benzene diazonium chloride, 2,5-diethoxy-4-(4''-methoxybenzoylamide)-benzene diazonium chloride and 4-N,N-ethylhydroxyethyl benzene diazonium chloride. 